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Ubiquinol
Ubiquinone (n = 6-10)](https://image.slidesharecdn.com/bio-energeticsand-170611084050/85/Oxidative-phosphorylation-71-320.jpg)
Uploaded byRamesh Gupta
Oxidative phosphorylation
The document discusses the biochemical processes through which energy is derived from macronutrients (carbohydrates, lipids, and proteins) and converted into adenosine triphosphate (ATP) through oxidative phosphorylation. It covers key components of the respiratory chain, various high-energy and low-energy phosphates, and the role of enzymes involved in biological oxidation. Additionally, it explains the mechanisms of substrate-level and oxidative phosphorylation as methods of ATP generation in cellular respiration.
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Oxidative phosphorylation
- 1. R. C. Gupta Professorand Head Dept. of Biochemistry National Institute of Medical Sciences Jaipur, India
- 2. Energy We require energyfor various activities: Muscle contraction Nerve conduction Synthetic reactions Active transport EMB-RCG
- 3. The ultimate sourceof energy is the food that we consume Carbohydrates, lipids and proteins present in food provide us energy These are present in food in the form of large complex molecules
- 4. Complex molecules in food Mono- saccharides, fatty acids andamino acids Oxidation in catabolic pathways The carbon atoms are oxidized to carbon dioxide Hydrogen atoms are transferred to coenzymes e.g. NAD+, FMN, FAD etc EMB-RCG
- 5. Reduced coenzymes transferthe hydrogen atoms to the mitochondrial respiratory chain wherein these are oxidized to water The energy released during this oxidation is used to phosphorylate adenosine diphosphate (ADP) to adenosine triphosphate (ATP)
- 6. Oxidation coupled withphosphorylation of ADP is known as oxidative phosphorylation This is the mechanism by which the energy present in various nutrients is captured in an easily utilizable form
- 7. Energy released duringoxidation of nutrients may not be required immediately High-energy phosphates The energy released during catabolism is captured in the form of “high-energy phosphates” There must be some way of storing energy so that it may be readily available when needed
- 8. The most importanthigh-energy phosphate is ATP Hydrolysis of ATP into ADP and Pi liberates 7.3 kcal of energy per mol Hydrolysis of ADP into AMP and Pi also releases nearly the same amount of energy
- 9. However, hydrolysis ofAMP into adenosine and Pi liberates much less energy (3.4 kcal/mol) This difference is because of the nature of bonds by which phosphate is attached
- 10. In ATP: Third phosphateis attached to the second by acid anhydride bond Second phosphate is attached to the first by acid anhydride bond The first phosphate is attached to ribose by an ester bond
- 11. H CH2— O —P— O — P — O — P — OH OH H OH H H O || O || O || | OH | OH | OH Acid anhydride bond Ester bond Acid anhydride bond Adenine O Adenosine triphosphate
- 12. Hydrolysis of acidanhydride bonds releases much more energy than hydrolysis of ester bonds Lipmann suggested a curved line (~) to denote a high-energy bond Thus, ATP may be represented as: Adenosine− P ~ P ~ P
- 13. Compound D GoCompound D Go Phosphoenol pyruvate – 14.8 AMP (Adenosine + Pi) – 3.4 Carbamoyl phosphate – 12.3 Glucose-1-phosphate – 5.0 1,3-Biphosphoglycerate – 11.8 Fructose-6-phosphate – 3.8 Creatine phosphate – 10.3 Glucose-6-phosphate – 3.3 ATP (ADP + Pi) – 7.3 Glycerol-3-phosphate – 2.2 Standard free energy (DGo) of hydrolysis of some important organic phosphates (kcal/mol)* *These are the values of DGo obtained in standard laboratory conditions of 1M reactant concentration at pH 7.0 at 25°C; values obtained in living cells (DG’o) are different as the reactant concentrations, pH and temperature are different
- 14. Compounds which liberate6 kcal/mol or more on hydrolysis of the phosphate group are known as high-energy phosphates Organic phosphates which release less than 6 kcal/mol on hydrolysis of the phosphate group are known as low-energy phosphates
- 15. High-energy phosphates ATP andthe related nucleotides Phosphoenol pyruvate Carbamoyl phosphate 1,3-Biphospho- glycerate Creatine phosphate Low-energy phosphates AMP Glucose-6-phosphate Glucose-1-phosphate Fructose-6- phosphate Glycerol-3-phosphate EMB-RCG
- 16. Compounds having DGoabove that of ATP can transfer their phosphate to ADP forming ATP The compounds having DGo below that of ATP cannot transfer their phosphate groups to ADP
- 17. Adenosine triphosphate Energy currencyof the cells Captures energy from exergonic reactions Can transfer it to endergonic reactions Storage form of energy EMB-RCG
- 18. Besides high-energy phosphates,some sulphur compounds having thio-ester bonds are also high-energy compounds In acetyl CoA, succinyl CoA, acyl CoA etc, CoA is attached by a high-energy thio- ester bond R‒C~S‒CoA O
- 19. Oxidation was definedin the past as addition of oxygen to or removal of hydrogen from a substance The reverse was termed as reduction These definitions have now been supplanted by a more comprehensive concept Oxidation and reduction
- 20. Oxidation is nowdefined as removal of electrons and reduction as addition of electrons The electron being removed or added may be a free electron or it may be associated with a proton as in a hydrogen atom
- 21. Fe+2 Oxidation of Fe+2 Reductionof Fe+3 Fe+3 + e‒ (electron) AH2 + NAD+ AH2 + FAD Oxidation of AH2 Oxidation of AH2 Reduction of A Reduction of A A + NADH + H+ A + FADH2
- 22. Our earliest conceptsof oxidation in living beings (biological oxidation) originated from the work of Lavoisier He proposed that respiration in animals is an oxidative process In this, atmospheric oxygen is used to oxidize carbon atoms to carbon dioxide
- 23. Pasteur showed laterthat the presence of oxygen is not essential for oxidation He showed that living organisms can oxidize substrates even in the absence of oxygen Wieland proposed that biological oxidation occurred by dehydrogenation of activated substrates
- 24. With the discoveryof cytochromes by Keilin, it became clear that the substrates are dehydrogenated The reducing equivalents ( H or e–) are taken up by cytochromes They are finally transferred to oxygen in the presence of cytochrome oxidase (Warburg’s enzyme) to be converted into water
- 25. Electron transfer chain(ETC) Also known as respiratory chain Present in inner mitochondrial membrane Sequence of carriers of reducing equivalents Consists of enzymes and coenzymes Transfers reducing equivalents from substrates to oxygen EMB-RCG
- 26. Substrate Nicotinamide-linked dehydrogenase Flavin-linked dehydrogenase Cytochrome bCytochrome c1 Cytochrome cCytochrome a Cytochrome a3 Oxygen Coenzyme Q
- 27. A reactant undergoingreduction-oxidation can exist in reduced form and oxidized form The reduced and the oxidized forms of the reactant constitute a redox couple Every redox couple has a redox potential ‒ a measure of its affinity of for electrons Redox potential
- 28. A redox couplehaving high redox potential has a high affinity for electrons It readily accepts electrons from a redox couple having a lower redox potential The redox potential of a reactant can be measured in the laboratory
- 29. Reduced and oxidisedforms of reactant at 1M concentration each are taken in sample cell 1M solution of H+ in equilibrium with H2 gas is taken in a reference cell The two are connected by an agar bridge through which electrons can flow Measurement of redox potential
- 30. Electrodes dipping ineach solution are connected to a voltmeter The potential difference between the two solutions is the redox potential of the reactant
- 31.
- 32. For biological systems,redox potential (Eo) is measured at pH 7.0, and is denoted by E’o Components of respiratory chain are arranged in increasing order of redox potential The electrons move from a relatively electro-negative component to a relatively electro- positive component at every site
- 33. Redox couple E´o(volts) 2H+- H2 – 0.42 NAD+ - NADH – 0.32 FAD - FADH2 – 0.22 Cyt b(Fe+3) - Cyt b(Fe+2) – 0.08 CoQ - CoQH2 + 0.04 Cyt c1 (Fe+3) - Cyt c1 (Fe+2) + 0.22 Cyt c (Fe+3) - Cyt c (Fe+2) + 0.25 Cyt a (Fe+3) - Cyt a (Fe+2) + 0.29 Cyt a3 (Fe+3) - Cyt a3 (Fe+2) + 0.38 ½ O2 - H2O + 0.82 Redox potential (E’o) of carriers in the respiratory chain
- 34. The enzymes concernedwith biological oxidation are oxido-reductases They can be sub-divided into: (i) oxidases, (ii) dehydrogenases (iii) hydroperoxidases (iv) oxygenases Enzymes involved in biological oxidation
- 35. Oxidases transfer hydrogenfrom a substrate to oxygen forming water or hydrogen peroxide The enzymes forming water are metallo- enzymes that usually contain copper e.g. cytochrome oxidase and tyrosinase The general reaction catalysed by oxidases is: AH2 + ½ O2 → A + H2O Oxidases
- 36. The enzymes forminghydrogen peroxide are flavoproteins containing FMN or FAD Examples are L-amino acid oxidase and xanthine oxidase These enzymes catalyse the general reaction: AH2 + O2 → A + H2O2
- 37. These are conjugatedproteins containing nicotinamide nucleotides or flavin nucleotides or iron-porphyrin They remove hydrogen from a substrate, and transfer it to another substrate The prosthetic group acts as carrier of hydrogen atoms Dehydrogenases
- 38.
- 39. Prosthetic group Examples NADLactate dehydrogenase, malate dehydrogenase etc NADP Glucose-6-phosphate dehydro- genase, 6-phosphogluconate dehydrogenase etc Flavin nucleotides Succinate dehydrogenase, acyl CoA dehydrogenase etc Iron- porphyrin Cytochrome a, cytochrome b etc
- 40. These enzymes convertH2O2 into H2O, and include peroxidase and catalase Peroxidase catalyses the reaction: H2O2 + AH2 A + 2 H2O Catalase catalyses the reaction: 2 H2O2 O2 + 2 H2O Hydroperoxidases
- 41. These enzymes incorporateoxygen into a substrate They can be sub-divided into: Oxygenases Mono-oxygenasesDi-oxygenases
- 42. These enzymes catalysethe incorporation of both the atoms of O2 into the substrate: A + O2 → AO2 Examples include homogentisate oxidase and tryptophan pyrrolase
- 43. These enzymes incorporateone atom of oxygen molecule into the substrate The other atom of oxygen oxidises another reduced substrate to water: AH + O2 + BH2 → A–OH + B + H2O Mono-oxygenases
- 44. Mono-oxygenases are alsoknown as hydroxylases or mixed function oxidases They are used to metabolize xenobiotics (foreign compounds), for example drugs like morphine, rifampicin etc They also hydroxylate endogenous substrates e.g. phenylalanine, tryptophan, cholecalciferol, steroids etc
- 45. Two slightly differenthydroxylase systems are present in the cells: Microsomal hydroxylase system Mitochondrial hydroxylase system
- 46. Microsomal hydroxylase system Completesystem for hydroxylation of xenobiotics and some steroids Present in microsomes (membrane of endoplasmic reticulum) EMB-RCG
- 47. Microsomal hydroxylase systemconsists of: Cytochrome P-450 (CYP) which possesses mono-oxygenase activity NADPH NADPH:cytochrome P-450 reductase (a flavoprotein containing FAD and FMN) Molecular oxygen EMB-RCG
- 48. NADPH ‒ Cytochrome P‒ 450 reductase Cytochrome P ‒ 450 (mono-oxygenase) Microsomal hydroxylase system EMB-RCG
- 50. A‒OH + H2O e- e-2H+ AH AH AH AH AH AH
- 51. Present in innermitochondrial membrane Catalyses hydroxylation of endogenous substrates The reaction catalysed by this system is similar to microsomal hydroxylation reaction Mitochondral hydroxylase system
- 52. The enzyme isNADPH:adrenodoxin reductase (FAD-containing flavoprotein) instead of NADPH:cytochrome P-450 reductase The reducing equivalents accepted by FAD are transferred to cytochrome P-450 via adreno- doxin (iron-sulphur protein)
- 53. Iron-sulphur centres Adrenodoxin possessesan iron-sulphur centre having catalytic activity Cys Cys Cys Cys Cys Cys CysCys
- 54. NADPH‒ Adrenodoxin reductase Cytochrome P‒450 (mono- oxygenase) Iron- sulphur protein AH +O2 A-OH + H2O Mitochondral hydroxylase system FAD Fe+3FeS EMB-RCG NADPH + H+ NADP+
- 55. Oxidative phosphorylation Process bywhich energy released during oxidation of energy-rich substrates is used to phosphorylate ADP Can occur at the substrate level or at the level of the respiratory chain EMB-RCG
- 56. Substrate level Oxidation of substrate is linkedwith formation of a high-energy bond in the product which, in turn, is used to convert ADP into ATP EMB-RCG Respiratory chain ADP is phosphorylated when the reducing equivalents removed from various substrates are oxidized in the respiratory chain
- 57. • Also knownas substrate-linked oxidative phosphorylation • Examples are found in the glycolytic pathway and citric acid cycle Oxidative phosphorylation at substrate level EMB-RCG
- 58. H ‒ C= O H ‒ C ‒ OH CH2 ‒ O ‒ P O II Glyceraldehyde-3-phosphate Glyceraldehyde-3- phosphate dehydrogenase NAD+ + Pi NADH + H+ C ‒ O ~ P CH2 ‒ O ‒ P H ‒ C ‒ OH 1, 3-Biphosphoglycerate I I In glycolysis, oxidation of glyceraldehyde-3- phosphate is linked with introduction of a high-energy phosphate in the product
- 59. 1, 3-Biphosphoglycerate 3-Phosphoglycerate Phospho- glycerate kinase In thenext reaction, the high energy phosphate is transferred to ADP forming ATP
- 60. In another glycolyticreaction, energy released during dehydration of 2-phosphoglycerate converts a low-energy phosphate into a high- energy phosphate The high-energy phosphate is transferred to ADP in the next reaction
- 61. H COOH C O P CH2OH COOH H2O CO CH2 CH2 COOH OHC ADP ATP Enolase Enol pyruvate Phosphoenol pyruvate 2-Phospho- glycerate Pyruvate kinase P
- 62. CH2 — COOH | CH2— C — COOH CH2 — COOH | NADH+H+ + CO2 a-Ketoglutarate dehydrogenase CH2 — C ~ S — CoA O a-Ketoglutarate Succinyl CoA NAD+ + CoA‒SHO In citric acid cycle, the energy released during oxidation of a-ketoglutarate is used to form a high-energy bond between succinate and CoA
- 63. CH2 — COOH CH2— COOH| | CH2 — COOHGDP + Pi GTP + CoA–SH Succinate thiokinase CH2 — C ~ S — CoA O Succinyl CoA Succinate In the next reaction, CoA is split off and the energy released is used to phosphorylate GDP to GTP
- 64. Only a smallfraction of energy is captured by substrate-linked oxidative phosphorylation For example, complete oxidation of one glucose molecule phosphorylates 38 molecules of ADP Of these, only six are phosphorylated at the substrate level The rest of the energy is captured in the respiratory chain
- 65. Energy-giving nutrients areoxidized stepwise by a series of reactions in various metabolic pathways In many reactions, reducing equivalents are removed from the substrates, and are taken up by coenzymes like NAD and FAD Oxidative phosphorylation at the level of respiratory chain
- 66. The reduced coenzymestransfer the reducing equivalents to respiratory chain Reducing equivalents are oxidized in the respiratory chain Their oxidation is coupled with the phosphorylation of ADP to ATP
- 67. This is themost important mechanism for capturing the energy present in various nutrients As the respiratory chain is located in mitochondria, the mitochondria are called the power-house of the cells
- 68. Substrate Nicotinamide-linked dehydrogenase Flavin-linked dehydrogenase Cytochrome bCytochrome c1 Cytochrome cCytochrome a Cytochrome a3 Oxygen Coenzyme Q Sequence of carriers in the respiratory chain
- 69. Most of thesubstrates transfer the reducing equivalents to NAD Reduced NAD transfers them to a flavoprotein containing FMN and iron-sulphur (FeS) centre Reduced flavoprotein transfers the reducing equivalents to coenzyme Q (ubiquinone) Transport of reducing equivalents in respiratory chain
- 70. Coenzyme Q isa fat-soluble compound resembling vitamin K in structure It contains 6-10 isoprenoid units, and can be reduced to ubiquinol
- 71.
- 72. Reduced coenzyme Qtransfers the reducing equivalents to cytochrome b Cytochrome b is associated with iron- sulphur protein The subsequent carriers, cytochromes c1, c and a, are typical iron-porphyrin proteins
- 73. The cytochromes differfrom each other in their protein portions and in the side chains attached to the porphyrin nucleus The cytochromes transport electrons
- 74. Prosthetic group ofcytochrome a
- 75. Prosthetic group of cytochromeb Prosthetic group of cytochrome c
- 76. The electrons aretaken up and transferred by the iron portion of the cytochromes Iron oscillates between Fe+3 and Fe+2 forms The last cytochrome (cyt a3) is an oxidase It catalyses the transfer of reducing equi- valents to oxygen forming water
- 77. QAH2 A NAD+ NADH + H+ (FMN, FeS) Fp (Cyt b, FeS) (Cytc1) (Cyt c) (Cyt a) (Cyt a3) ½ O2 H2O Succinate Fp (FAD, FeS) 2H+ FpH2 QH2 2Fe+2 2Fe+3 2Fe+3 2Fe+3 2Fe+2 2Fe+2 2Fe+2 2Fe+3 2Fe+2 2Fe+3
- 78. Components of respiratorychain do not function as discrete carriers of reducing equivalents They are organized into four complexes Each complex acts as a specific oxido- reductase
- 79. Coenzyme Q andcytochrome c are not parts of any complex They are not fixed in the inner mitochondrial membrane The other components are fixed in the membrane
- 80. Succinate FAD, FeS Complex II NADHFMN,FeS Q Cyt b,FeS,Cyt c Cyt c Cyt aa3 O2 Complex I Complex III Complex IV
- 81. Complex I, II,III and IV in respiratory chain EMB-RCG
- 82.
- 83. Complex I actsas NADH:ubiquinone oxidoreductase, and transfers reducing equivalents from NADH to CoQ
- 84. Complex II actsas succinate:ubiquinone oxidoreductase, and transfers reducing equivalents from succinate to CoQ
- 85. Complex III actsas ubiquinol:ferricytochrome c oxidoreductase, and transfers reducing equivalents from reduced CoQ to cytochrome c
- 86. Complex IV actsas ferrocytochrome c:oxygen oxidoreductase,and transfers reducing equivalents from reduced cytochrome c to oxygen
- 87. The proximal endof the respiratory chain has a negative redox potential The distal end has a positive redox potential Electrons move from a relatively electro- negative component to a relatively electro- positive component Energy is released during this movement
- 88. Quantum of energyreleased at any site is proportional to the difference in redox potentials of: Thus, different quanta of energy are released at different sites Component accepting the reducing equivalents Component donating the reducing equivalents
- 89. Hydrolysis of theterminal phosphate of ATP releases 7.3 kcal of energy per mol in standard laboratory conditions But in conditions prevailing in living cells, the energy required for phosphorylation of ADP to ATP is about 10 kcal/mol
- 90. Quantum of energyreleased exceeds 10 kcal/mol if the difference in the redox potentials of the donor and the acceptor is 0.3 volts or more ADP is phosphorylated at these sites There are three such sites in the respiratory chain
- 91. Site I isbetween NAD and CoQ Site II is between Co Q and cytochrome c Site III is between cytochrome c and oxygen These sites correspond to complexes I, III and IV respectively in the respiratory chain
- 92. All the substratesundergoing dehydrogenation do not transfer the reducing equivalents to NAD Reducing equivalents are accepted by a carrier having a redox potential just above that of the substrate
- 93. Substrates that transferreducing equivalents to NAD can phosphorylate three molecules of ADP Examples are isocitrate, malate, glutamate etc
- 94. Substrates that transferreducing equivalents to FAD can phosphorylate only two molecules of ADP Examples are succinate, glycerol-3- phosphate, acyl CoA etc
- 95. The ratio ofADP molecules phospho- rylated to number of oxygen atoms reduced is known as P:O ratio P:O ratio is three when the reducing equivalents are accepted by NAD P:O ratio is two when the reducing equivalents are accepted by FAD
- 96. Mechanism by whichoxidation and phospho- rylation are coupled remained unclear for long Several hypotheses were advanced to explain the mechanism of this coupling The chemiosmotic hypothesis is the most plausible Mechanism of oxidative phosphorylation
- 97. Proposed by Mitchell Innermitochondrial membrane is impermeable to protons (H+) Energy released during transport of electrons is used to actively eject H+ from the matrix of mitochondria This ejection establishes an electro- chemical gradient across the membrane Chemiosmotic hypothesis
- 99. Complexes I, IIIand IV in the respiratory chain act as proton pumps ejecting hydrogen ions from the mitochondrial matrix to the inter-membrane space Succinate Fumarate
- 100. The concentration ofH+ on the outer side becomes higher as compared to the inner side The outer side also becomes electro- positive as compared to the inner side
- 101. This electrochemical gradientincreases up to a certain limit When this limit is reached, the hydrogen ions re-enter the matrix releasing energy
- 102. Re-entry of protonsreleases energy
- 103. The energy releasedduring influx of protons is used to activate a membrane-bound enzyme This enzyme, vectorial ATP synthetase, converts ADP and Pi into ATP
- 104. Efraim Racker showedthat: Vectorial ATP synthetase is made up of F0 and F1 components F0 component is embedded in the inner mitochondrial membrane F1 component projects into the matrix
- 105.
- 106.
- 107. F0 component actsas a channel for the passage of hydrogen ions F1 component has got ATP synthetase activity This activity is switched on when hydrogen ions pass through the F0 component
- 108. John Walker decipheredthe genes encoding the F0 and F1 components The F0 component is made up of a, b and c subunits The F1 component is made up of a, b, g, d and e subunits (a3b3gde)
- 109. The subunits ofF0 and F1 components of vectorial ATP synthetase EMB-RCG
- 110. This form readilycombines with ADP by a high-energy bond Water is formed and inorganic phosphate is converted into a highly reactive form Two hydrogen ions combine with two electrons and one oxygen atom of inorganic phosphate When hydrogen ions flow back into the matrix: EMB-RCG
- 111. O || || O P‒O‒ O || O || | | –O –O H2OH2O3 O || O‒P‒O‒ – O || O || | O | | –O –O Pi ADP 2H+ 4 | ATP 2H+ O‒P‒O‒P‒O‒ A‒ ‒ ‒ O‒P‒O‒P‒O‒P‒O‒ A O || O || | | –O –O || O O– – O‒P‒O‒P‒O‒ A
- 112. Paul Boyer hasshown that: F1 complex has got three sites having specific conformations These are O site (open site), L site (loose-binding site) and T site (tight- binding site)
- 113. Each site ismade up of one a and one b subunit The three sites are inter-convertible In the absence of an electrochemical gradient: T site is occupied by ATP L site is occupied by ADP & Pi
- 114. T site isconverted into O site, the O site into L site and the L site into T site The energy released during the influx of H+ inter-converts the three sites Boyer proposed the binding-change mechanism EMB-RCG
- 115. Conversion of Tsite into O site releases the bound ATP Conversion of L site into T site converts the bound ADP & Pi into ATP Another pair of ADP & Pi enters the new L site, and the cycle is repeated EMB-RCG
- 116.
- 117. The inter-conversion ofsites occurs because of rotation of a and b subunits of F1 component The rotation occurs in steps of 120o
- 118. There is strongexperimental evidence in support of chemiosmotic hypothesis Building up of H+ gradient is the basic premise of the hypothesis It is seen that on bathing mitochondria in a fluid having a relatively high H+ concen- tration, phosphorylation occurs in the absence of oxidation
- 119. The hypothesis envisagesa membrane- bound vectorial ATP synthetase It has been shown that disruption of the mitochondrial membrane leads to loss of ATP synthetase activity
- 120. The P:H+ andH+:O ratios of various substrates are also in agreement with the experimental evidence Thus, the chemiosmotic hypothesis has now become the accepted theory to explain oxidative phosphorylation in the respiratory chain
- 121. Certain agents areknown to inhibit oxidative phosphorylation at specific sites in the respiratory chain Amobarbitone, rotenone and piericidin A inhibit oxidative phosphorylation at site I These have been shown to inhibit the oxidoreductase activity of complex I Inhibitors of oxidative phosphorylation
- 122. Dimercaprol and antimycininhibit the oxidoreductase activity of complex III H2S, carbon monoxide and cyanide inhibit the oxidoreductase activity of complex IV When oxidation is inhibited, phospho- rylation also cannot occur
- 123. Oligomycin inhibits oxidative phosphorylationat all the sites It binds to and inhibits F0 component of ATP synthetase The subscript ‘o’ derives from the tendency of this component to bind oligomycin
- 124. Inhibitors of OxidativePhosphorylation
- 125. Some compounds areknown to uncouple oxidation and phosphorylation Examples are dinitrophenol, dinitrocresol and dicoumarol In their presence, phosphorylation is inhibited but oxidation goes on Uncouplers of oxidative phosphorylation
- 126. The uncouplers makethe inner mito- chondrial membrane permeable to H+ This doesn’t allow electrochemical gradient to build up Therefore, ATP cannot be synthesized even though oxidation is going on
- 127. Thermogenin is aprotein present in brown adipose tissue Brown adipose tissue is rich in mitochondria Thermogenin is present in inner mito- chondrial membrane It acts as a channel for entry of hydrogen ions into mitochondria An endogenous uncoupler
- 128.
- 129. As hydrogen iongradient can not build up, phosphorylation does not occur Oxidation occurs in brown adipose tissue without generation of ATP resulting in production of heat Brown adipose tissue is present in significant amount in infancy and decreases with age
- 130. Oxidative phosphorylation inthe respiratory chain results in consumption of oxygen This is also known as tissue respiration When oxidizable substrates and oxygen are available, the rate of tissue respiration is regulated mainly by concentration of ADP Regulation of oxidative phosphorylation
- 131. Increased utilization ofATP raises ADP concentration This stimulates tissue respiration resulting in increased phosphorylation of ADP into ATP
- 132. Most of theNADH is produced in mitochondria Some NADH is produced in cytosol also Mitochondrial membrane is impermeable to NADH Special mechanisms are required to transport NADH from cytosol into mitochondria Oxidation of extra-mitochondrial NADH
- 133. Two important mechanismsare: Malate shuttle Glycero- phosphate shuttle EMB-RCG
- 134. Cytosolic NADH isused to reduce dihydroxyacetone phosphate to glycerol-3- phosphate The latter goes into mitochondria as the mitochondrial membrane is permeable to it In mitochondria, glycerol-3-phosphate is oxidized to dihydroxyacetone phosphate Glycerophosphate shuttle
- 135. Dihydroxyacetone phosphate comesout into the cytosol In the mitochondrial reaction, the hydrogen atoms are accepted by FAD Only two ADP molecules are phosphorylated when FADH2 is oxidized in the respiratory chain
- 136.
- 137. This is quantitativelymore significant Cytosolic malate dehydrogenase (MDH) reduces oxaloacetate to malate NADH is converted into NAD in this reaction Malate shuttle
- 138. Malate goes intomitochondria, and is oxidized to oxaloacetate by mitochondrial MDH The reducing equivalents are accepted by NAD; hence there is no loss of energy But mitochondrial membrane is not permeable to oxaloacetate
- 139. For transporting oxaloacetateback to cytosol, a special mechanism is needed Mitochondrial glutamate oxaloacetate transaminase (GOT) catalyses a trans- amination reaction It transfers an amino group from glutamate to oxaloacetate forming a- ketoglutarate and aspartate
- 140. a-Ketoglutarate and aspartatecome out of mitochondria They are reconverted into glutamate and oxaloacetate by cytosolic GOT Glutamate goes back into mitochondria
- 141. Cytosol Mitochondrial Matrix NADH+H+ MDH Oxalo- acetate Oxalo- acetateAspartateAspartate NADH+H+ 3 3 GOT Glutamate NAD+ NAD+Malate a-Keto- glutarate a-Keto- glutarate MDH Glutamate Malate4 GOT 5 Malate shuttle
- 142. The mitochondrial membraneis selectively permeable Small uncharged molecules can pass through the membrane easily Monocarboxylic acids can also pass through the membrane Transport across mitochondrial membrane
- 143. Mitochondrial membrane isimpermeable to: Special transport mechanisms are required to transport these Amino acids Tricarboxylic acids Dicarboxylic acids
- 144. Two compounds traversing themembrane in the same direction Two compounds traversing the membrane in opposite directions The transport mechanisms may operate as:
- 145. Transport of pyruvateand H+ into mitochondria is an example of a symport Membrane Mitochondrial matrixCytosol Pyruvate Pyruvate H+ H+
- 146. Memb MatrixCytosol ADP ADP ATPATP Malate a-Ketoglutarate Malate Malate Citrate + H+ + + Glutamate Glutamate Aspartate Aspartate Some important antiports Malate a-Ketoglutarate Citrate + H+